Hundreds of thousands of protesters took to streets in cities across the world on January 21 to show solidarity with a Washington, D.C., protest dubbed the "Women's March" against newly inaugurated U.S. President Donald Trump.

At the main demonstration in Washington, large crowds of women and men poured into the downtown area by bus, train, and car from across the country.

Many marchers wore bright pink hats as they gathered to listen to speakers at the event.

One speaker told the crowd that 500,000 people gathered for the Washington event, although that claim was not independently confirmed.

The Associated Press reported that, by early afternoon in Washington, crowds had packed the entire route of the planned march -- preventing organizers from leading a journey on foot to the White House.

March organizers, however, said that the procession would still go ahead.

With police and Secret Service personnel standing by, protesters eventually made it to about a block from the White House, held back by metal barriers. By early evening, D.C. police said there had been no march-related arrests.

As demonstrators rallied alongside the National Mall, Trump began his first full day as president by attending a prayer service at the Washington National Cathedral.

On Trump's way back to the White House, his motorcade passed several large groups of protesters that he would have been hard-pressed to miss.

The pop star Madonna made a surprise appearance at the Washington protest, performing two songs and describing Trump's inauguration as "this horrific moment."

During her speech and performance, which were carried live on cable television, Madonna used several obscenities to emphasize her opposition to Trump.

Documentary filmmaker Michael Moore expressed amazement at the size of the crowd.

Moore then urged demonstrators to make telephone calls to their representatives in Congress on a daily basis as part of a "new daily routine."

'Women Won't Back Down'

Feminist activist Gloria Steinem warned demonstrators about the possibility of a "Twitter finger becoming a trigger finger" -- a reference to Trump's activity on the social network and her concerns that the United States could become involved in fresh military conflicts under Trump's presidency.

Steinem also relayed a message from so-called "sister" marches in Germany, saying demonstrators there had sent the message: "We in Berlin know that walls don't work" -- a reference to the Berlin Wall of the Cold War-era and Trump's campaign pledge to build a wall along the entire southern U.S. border with Mexico.

The flood of demonstrators coming into and out of the U.S. capital overwhelmed Washington's Metro transport system, with 597,000 riders reported by 4 p.m. local time on January 21.

At the same time on January 20 for Trump's inauguration, there were 368,000 trips recorded.

Organizers initially said they expected 200,000 people to attend their Washington march, which began at 10 a.m. local time, and more than 2 million to attend protests worldwide.

PHOTO GALLERY: Women Of The World March Against Trump

Photo Gallery:

Women Of The World March Against Trump

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They said they want their voices to be heard by Trump's administration on his first full day in office following his January 20 inauguration.

"The Women's March on Washington will send a bold message to our new government on their first day in office, and to the world, that women's rights are human rights," they said on the womensmarch.com website. "We stand together, recognizing that defending the most marginalized among us is defending all of us."

Besides promoting equal rights for women, the marchers also said they want to defend marginalized groups -- including people of color, ethnic and religious minorities, people with disabilities, and the LGBT community.

The first protests on January 21 began in Australia and New Zealand, with several thousand women and men marching in Sydney, Melbourne, Wellington, and Auckland.

With rallies planned in 673 locations around the world, marchers also came out in London, Prague, Dublin, Amsterdam, Paris, Seoul, Tokyo, and other cities.

Organizers urged those participating to protest peacefully.

"Nonviolence is a way of life for courageous people," they said.

Women's March Sydney co-founder Mindy Freiband told protesters in Australia's largest city: "Hatred, hate speech, bigotry, discrimination, prejudicial policies -- these are not American problems, these are global problems,"

'Grassroots Movement'

In London, organizers called for an international day of solidarity and said more than 100,000 demonstrators joined the demonstration there -- although there was no independent confirmation of that estimate.

"The U.S. election proved a catalyst for a grassroots movement of women to assert the positive values that the politics of fear denies," they said. "We, the organizers of the London march, call on people of all genders to march in London as part of an international day of action in solidarity."

In Paris, several thousand people gathered at the Eiffel Tower as part of the protest.

Police in Switzerland said 2,000 marchers joined the demonstration in Geneva, more than the 1,000 that had been expected by Geneva protest organizer Karen Olson.

Olson, who was also working as a global coordinator for the demonstrations, said organizers wanted to "create a platform for everyone who agrees that women's rights are human rights and that we must be vigilant to protect the human rights and civil rights" within a "climate of uncertainty created by the rise of populism around the world that is acting as a vehicle for racism, sexism, bigotry, xenophobia, nationalism, and isolationism."

Olson said the "key values" of the demonstrators are "equality, diversity, and inclusion with a focus on the cross-cutting issue of climate which affects us all."

She said Geneva demonstrators "understood that the old formula 'Think Global, Act Local' [has] now [been] updated to 'Mobilize Global, Organize Local'."

In Moscow, organizers also staged a solidarity march for January 21.

They said on the actionnetwork.org website that the Moscow march was a local event for those unable to travel to Washington, D.C. to show their solidarity.